CHEMISTRY 2351
 
Assignment #2 - solutions


1.
trans point group is D3h
trans (left picture)
trans (right picture)
 E  C3  C2  σh  S3  σv
1    1    1    1    1    1     = A1' = IR active only
1    1   -1   -1   -1   1     = A2" = Raman active only
 
cis point group is C2v
cis (left picture)
cis (right picture)
 E   C2   σv (xz)   σv' (yz)
1    -1     1             -1          = B1 = active in both
1    -1     -1             1          = B2 = active in both

 
2. (a)
 -  F heavier than O, therefore unlikely to see mixing, therefore H 1s can only interact with either F 2s or F 2p
-  the energy match between H 1s and F 2p is closest in energy, therefore, that is where the interaction is
 
2. (b)
 - N will have mixing, therefore you get:
2
 
2. (c)
 Two lone pairs are in the px and py orbitals which are at 90o angles to each other, therefore, they are not tetrahedrally oriented.
 
3.
 The MO picture for C2 would be (with mixing!):
3
 
3. (a)
 The bond order (for C2) is 3, if you count the σu(nb) as a non-bonding orbital.
 
3. (b)
 The bond order of C22- is still 3, because the extra 2 electrons went into a non-bonding orbital.
 
3. (c)
 The HOMO is an orbital that has electron density pointing out at 180o angles from the C-C bond.  Therefore, because this is the orbital with the correct linear geometry and because the energetics will be favourable, the HOMO will interact with H+ to form a C-H bond.  (It turns out that the σu(nb) is also used for C-H bonding - to form two C-H bonds requires two M.O.'s on the C22- fragment.)


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